Final answer:
Ailerons are control surfaces that control the roll of the aircraft about its longitudinal axis.
Step-by-step explanation:
The flight controls attached to the wings and tail assembly of an aircraft are called ailerons.
Ailerons are control surfaces that are located on the trailing edge of the wings and are used to control the roll of the aircraft about its longitudinal axis. By moving the ailerons up or down, the pilot can increase or decrease the lift on one wing, causing the aircraft to roll in the desired direction.
Hence, the correct option is (a) control roll about the longitudinal axis.